Magnetic resonance tomography (MRT) devices are well known in the state of the art. An element of a MRT device is a magnet defining, by its magnetic field, a center frequency for the radiation emitted by a patient or a probe being located in the MRT, wherein the radiation is emitted after an excitation by a high frequency pulse. Corresponding to the center frequency, a receiving coil of the MRT-device is configured to receive the emitted radiation.
For superconducting magnets, one cryogenic solution is to cool the magnet in a Helium bath where the magnet coils are enclosed in vessel in direct contact with liquid Helium. Most systems use around 1000-2000 liquid liters of Helium, which adds cost due to the complexity of managing the Helium inventory. One of the current trends for superconducting magnets is to dramatically reduce (e.g., Low Helium Inventory—LHI) or remove the Helium system (e.g., Dry Magnet) from the magnet. This has already been seen on small magnets used for NMR and animal MRI. Removing the Helium system presents many technical challenges and practical challenges of having such a magnet operate in a real hospital or clinic environment. In particular, for “LHI and dry magnets”, where there is a probability for a breakdown of the site electric power supply and/or a breakdown of a cooling supply for the magnet refrigeration system, there is the risk of a “quench” where the magnet coils become resistive leading to a rapid reduction of field and warming of the magnet, leading to a lengthy recovery time and customer downtime. To reduce the downtime of the MR system for the customer, the MRT-device may be run-down to zero field in such scenarios, in particular, by using a ramp-down mode transferring the magnet from an operating status into a non-operating status but avoiding the lengthy recovery time after a quench. Once the magnet refrigeration has been restored, the magnet is transferred from the non-operating state to the operating state by re-cooling the magnet and re-ramping the magnet to field. However, this reestablishing of the magnetic field might result in a shift of the magnetic field established at the end of the ramp-up mode compared to the previous realized magnetic field. A resulting mismatch between the center frequency of the emitted radiation and a center frequency of the receiving coil causes a reduction of a transmission performance of the MRT-device.
PCT Publication No. WO 2014/199793 A1 describes a method for restarting a magnet of a MRT during an operation of a refrigerator. In particular, it is provided to increase a current of a superconducting coil until the current value reaches the current value in a state of a previous operation of the coil.
U.S. Patent Application Publication No. 2005/0111159 A1 concerns another technique for placing superconducting magnets into operation. For example, the technique provides for automatically controlling ramp-up of a superconducting magnet. In one aspect, the technique includes connecting a power supply to the magnet, determining constraining parameters of the ramp-up automatically, applying power to the magnet, automatically controlling the ramp-up based on the constraining parameters, and wherein the ramp-up is complete upon reaching a predetermined value of a target parameter.